The first day of multiple-state voting looms large in a wild presidential race after early states trimmed the field and the brash billionaire and his army of outsider voters are positioned to send panic through the Republican establishment by tightening his grip on the party's nomination.

Both Trump and Clinton head into the most important day yet in the 2016 election dominating their respective races. A CNN/ORC national poll out Monday shows Trump getting 49% of the Republican primary vote -- 30 percentage points ahead of Florida Sen. Marco Rubio. On the Democratic side, Clinton tops Sanders 55% to 38%.

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The billionaire is, however, struggling to shake off a controversy after he failed to disavow former Ku Klux Klan grand wizard David Duke during an interview on CNN's "State of the Union" on Sunday. Trump blamed a bad earpiece for the oversight, and he has at other times disavowed Duke, but his rivals were quick to seize on the incident to suggest that he is unfit to be the Republican nominee.

Rubio said the remarks prove that the former reality show star is "unelectable."

"A disqualifying & disgusting response by @realDonaldTrump to the KKK. His coddling of repugnant bigotry is not in the character of America," tweeted Romney, who has been actively hitting Trump for days.

The contests on Tuesday, across 12 states, herald several weeks of nationwide skirmishes that will be decisive in determining who gets to face off for the White House in the fall.

The sheer scale of the battlefield favors Trump, whose ubiquitous media profile means he is known everywhere, and Clinton, whose decades in public life give her an advantage over the lesser-known Sanders.

Businessman Donald Trump announced June 16 at his Trump Tower in New York City that he is seeking the Republican presidential nomination. This ends more than two decades of flirting with the idea of running for the White House.

"So, ladies and gentlemen, I am officially running for president of the United States, and we are going to make our country great again," Trump told the crowd at his announcement.

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Photos:Who's running for president?

Sen. Ted Cruz of Texas has made a name for himself in the Senate, solidifying his brand as a conservative firebrand willing to take on the GOP's establishment. He announced he was seeking the Republican presidential nomination in a speech on March 23.

"These are all of our stories," Cruz told the audience at Liberty University in Virginia. "These are who we are as Americans. And yet for so many Americans, the promise of America seems more and more distant."

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Ohio Gov. John Kasich joined the Republican field July 21 as he formally announced his White House bid.

"I am here to ask you for your prayers, for your support ... because I have decided to run for president of the United States," Kasich told his kickoff rally at the Ohio State University.

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Hillary Clinton launched her presidential bid on April 12 through a video message on social media. The former first lady, senator and secretary of state is considered the front-runner among possible Democratic candidates.

"Everyday Americans need a champion, and I want to be that champion -- so you can do more than just get by -- you can get ahead. And stay ahead," she said in her announcement video. "Because when families are strong, America is strong. So I'm hitting the road to earn your vote, because it's your time. And I hope you'll join me on this journey."

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Sen. Bernie Sanders, an independent from Vermont who caucuses with Democrats, announced his run in an email to supporters on April 30. He has said the United States needs a "political revolution" of working-class Americans to take back control of the government from billionaires.

"This great nation and its government belong to all of the people and not to a handful of billionaires, their super PACs and their lobbyists," Sanders said at a rally in Vermont on May 26.

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It's Trump who may stand tallest on Tuesday night.

"On Tuesday, you have a big day," Trump told supporters at a big rally in Tennessee on Saturday, saying he didn't care if someone was at death's door or if their wife was leaving them -- they had to vote.

"You get up, you go to the polls, and you vote!" he said. "I promise you, that you are going to look back on this night and you are going to say this was a very important night ... a very important evening in your life."

While Trump expects to savor a night of triumph, his top rivals, Rubio and Texas Sen. Ted Cruz, have lesser expectations. Rubio is desperate to at last secure an elusive win, somewhere, anywhere. And Cruz faces a make-or-break moment in his home state of Texas.

A total of 595 Republican delegates are up for grabs of the 1,237 needed to clinch the GOP nomination. Sanders and Clinton are facing off for 865 of the 2,383 delegates needed to win the Democratic race.

Democrats will award delegates in the same states as Republicans, apart from Alaska, and they are also competing in Colorado and in American Samoa.

A fateful moment

Super Tuesday is coming at a fateful moment in the Republican race. It has finally dawned on rival campaigns and party establishment figures that far from fading as many predicted, Trump -- after his three thumping wins in New Hampshire, South Carolina and Nevada -- will clinch the nomination unless there is a dramatic twist in the race.

That realization spurred bitter clashes between Trump and Rubio on Friday and over the weekend and has Cruz also gunning for the billionaire. And it played into former candidate Chris Christie's endorsement of the erstwhile reality star he had once billed as unfit for the presidency.

Republican leaders and operatives, meanwhile, are wondering whether the blitz against Trump by Rubio at CNN's debate in Houston on Thursday came too late to halt the billionaire businessman.

"I am not sure throwing the whole kitchen sink is going to make much difference in the trajectory of the race," said Ford O'Connell, a GOP strategist not currently working for any presidential candidate. "Political scientists are going to wonder for years why they didn't go after him a lot earlier."

Polling is sparse in some states, so it's difficult to be certain about the outcome everywhere. But it's possible that Trump could barnstorm to victories in as many as 10 contests and whip up an unstoppable tailwind.

His rivals are already bracing for a tough night.

"Nobody's going to win but Trump." Kasich, who is hoping to remain viable until the race turns north, told CNN's Sara Murray on Thursday.

He added, "If he continues with that momentum and powers through and wins everywhere on Super Tuesday, he could easily be unstoppable."

Rubio is on a high after a stellar debate performance last week in which he pulled off the most effective prosecution yet of the Republican front-runner. But the euphoria could be dampened by the Super Tuesday math.

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Despite the Republican establishment flocking to the Florida senator, seeing him as the strongest anti-Trump candidate, he has yet to win a single contest after four states held votes -- and it is not obvious where that might happen on Tuesday.

His best chances appear to come in states that have a mix of religious, conservative and highly educated white-collar voters.

That's why the Rubio campaign and his Super PAC, Conservative Solutions, have been buying television advertising in areas including the Washington media market that covers the populous Northern Virginia suburbs and the region around the state capital of Richmond, according to Federal Communications Commission records. Rubio spent the entire day in Virginia on Sunday.

Rubio supporters such as Wisconsin Rep. Sean Duffy are predicting he can win Minnesota as well.

"Wait 'til Tuesday. As the field narrows, there is a consolidation of votes that I think are opposed to Donald Trump and those are going to go to Marco Rubio. We are going to see that consolidation, I think, project itself in wins," Duffy said on CNN.

But even if he is left winless on Tuesday, Rubio can survive because his must-win home state primary in Florida is on March 15. In theory, he could lose all of the Super Tuesday contests and start to overtake Trump when states start doling out delegates on a winner-take-all basis in mid-March.

Make or break for Cruz

For Cruz, his win-or-go-home moment is now.

"If Cruz doesn't win Texas, it is game over for him," said Phillip Stutts, a Republican political consultant. "Rubio doesn't have to win, but Cruz has to."

Trump's big advantage going into Super Tuesday is that his opposition remains divided.

Photos:Moments from Ted Cruz's career

Photos:Moments from Ted Cruz's career

U.S. Sen. Ted Cruz, a Republican presidential candidate, speaks during a campaign rally at the Indiana State Fairgrounds on Monday, May 2.

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Photos:Moments from Ted Cruz's career

Cruz holds up the hand of Carly Fiorina at a campaign rally in Indianapolis on Wednesday, April 27. Cruz named Fiorina, a former presidential candidate, as his running mate.

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Cruz celebrates his Wisconsin primary win with his wife, Heidi, and Gov. Scott Walker in Milwaukee on Tuesday, April 5. Walker endorsed Cruz for the presidency.

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With his wife by his side, Cruz tours the Dane Manufacturing facility before speaking to workers in Dane, Wisconsin, on Thursday, March 24.

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Cruz speaks during the CNN Republican debate in Miami on Thursday, March 10.

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Cruz and his wife wave to the crowd at Liberty University after he announced his presidential candidacy in Lynchburg, Virginia, on March 23, 2015.

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Cruz speaks during the 2013 NRA Annual Meeting and Exhibits at the George R. Brown Convention Center on May 3, 2013, in Houston, Texas.

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Cruz poses with his wife, Heidi, and his daughters Caroline and Catherine.

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Cruz (left) fields questions from Bruce Rastetter at the Iowa Ag Summit on March 7, 2015, in Des Moines, Iowa. The event allows the invited speakers, many of whom are potential 2016 Republican presidential hopefuls, to outline their views on agricultural issue.

Sen. Patrick Leahy (right) escorts Loretta Lynch back from a lunch break as Cruz (left) sits nearby during her confirmation hearing before the Senate Judiciary Committee January 28, 2015, on Capitol Hill.

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Cruz greets supporters at the South Carolina Tea Party Coalition convention on January 18, 2015, in Myrtle Beach, South Carolina. A variety of conservative presidential hopefuls spoke at the gathering on the second day of a three-day event.

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Cruz (left) and then-Texas Governor Rick Perry stand together during a press conference at the front gate of Fort Hood about Iraq war veteran, Ivan Lopez, who killed three and wounded 16 before taking his own life on April 4, 2014, in Fort Hood, Texas.

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Photos:Moments from Ted Cruz's career

(Left to right) Sen. Lindsey Graham, Sen. John McCain and Cruz listen as President Barack Obama delivers the State of the Union address on January 28, 2014, in Washington, D.C.

Cruz listens to testimony during a Senate Judiciary Committee hearing on April 22, 2013, in Washington, D.C.

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Cruz holds a news conference to announce the plan to defund Obamacare on March 13, 2013.

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Cruz speaks at the CPAC on March 6, 2014, in National Harbor, Maryland.

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Then-Senate Republican Candidate and Texas Solicitor General Cruz speaks at the 'Patriots for Romney-Ryan Reception' on August 29, 2012, in Tampa, Florida.

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Then-Senate Republican Candidate and Texas Solicitor General Cruz speaks during the Republican National Convention at the Tampa Bay Times Forum on August 28, 2012.

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Then-Senate Republican Candidate and Texas Solicitor General Cruz speaks during the Republican National Convention in 2012.

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Cruz, Rubio and Kasich in some states are dividing up the anti-Trump vote between them, meaning no single candidate can unite opposition to Trump.

"Right now, they are all fighting each other while Donald Trump wraps up delegates. That's a problem -- it needs to be a two-man race," said Stutts.

For instance, in Virginia, Trump leads with 41% while Rubio is at 27%, Cruz is at 14% and both Kasich and former neurosurgeon Ben Carson have 7% each, according to a Monmouth University poll last week.

And in Massachusetts, it's Kasich who is inadvertently helping Trump -- sitting tied in a WBUR poll with Rubio at 19% -- well behind Trump at 40%.

Even if Trump does sweep the field on Super Tuesday, his nomination will not be assured, however, because in the GOP, all states that vote before March 15 must divide delegates among the competing candidates based on their share of the vote, as long as they reach certain thresholds in some states.

But the real estate mogul could carve out a strong delegate lead ahead of big-state winner-take-all primaries such as Ohio and Florida in two weeks, where first-place finishes could effectively make him the Republican nominee.

Cruz, who once looked like a strong competitor to Trump after he emerged victorious in Iowa, finds himself in a weakened position after failing to beat Trump or Rubio in last Saturday's South Carolina GOP primary. It was an especially stinging blow because it raised questions about his plan to dominate Southern states packed with ideological conservatives and evangelicals in the so-called SEC Primary.

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"Cruz has the most riding on Super Tuesday," said Stutts. "His whole strategy really banked heavily on the Southern strategy. Not only does he have to win Texas, he has to find another 'W' and take home the most delegates on Super Tuesday."

While Cruz leads most polls in Texas, he trails Trump elsewhere. The Yellowhammer conservative news website reported that the Texas senator had pulled out of events in Alabama over the weekend in a possible sign his Southern firewall was crumbling.

New polls in two key states on Monday had sobering news for Cruz. In the Monmouth University survey in Alabama, Trump earned the support of 42% of likely GOP primary voters, compared to 19% for Rubio and 16% for Cruz. Trump leads in Oklahoma with 35% in a poll from the same organization, ahead of Cruz at 23% and Rubio at 22%.

Cruz's team is confident, however, that their boss can rewrite the political narrative Tuesday.

"This is the day that we have spent (the) most time on," former Virginia Attorney General Ken Cuccinelli, a Cruz supporter, told CNN's Wolf Blitzer on Friday.

"We are going to win multiple states next Tuesday."

Should Cruz win in Texas -- the state he referred to last week as the "crown jewel" of Super Tuesday because of its 155 delegates -- there could also be an upside for Trump.

That's because a win there would give Cruz a rationale to stay in the race and keep Rubio from coalescing opposition against the Republican front-runner.

"I don't think it is bad news for Donald Trump if he loses Texas," said CNN political analyst David Gergen on Thursday. "If he wins most of the other states and loses Texas, he will have momentum coming out of it, and Cruz stays in the race. And he needs Cruz to stay in the race."

Clinton could see advantage

On the other side of the aisle, the Democratic Super Tuesday clash is not going to force either of the candidates out of the race however it ends.

But it could hand a clear advantage to Clinton as she seeks to exploit the Southern advantage that her campaign has long argued makes it impossible for Sanders to win the nomination.

The former secretary of state will be looking to engineer a sweep of the Deep South, Virginia and Texas and to also be competitive in states where Sanders, a Vermont senator, looks to have his best chance.

That could allow Clinton to build up a lead in delegates before the race heads to Northern and Midwestern states where the Sanders message of an economy rigged against American workers could provide her with a more irksome challenge.

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That turnout suggests that she will triumph in states voting Tuesday with similar demographic profiles, including Alabama, Arkansas, Georgia, Tennessee and Virginia. Her aides know that she cannot knock out Sanders on Tuesday, but they hope to end the night with a lead of around 100 earned delegates.

Sanders, meanwhile, made clear where his future lies, paying only passing attention to the South Carolina primary on Saturday while spending the day in Texas and Minnesota.

His campaign is making clear that despite the size of his defeat in South Carolina -- equal to nearly 50% of the vote -- he is nowhere near giving up his campaign, though the candidate himself appears to have rock-bottom expectations in the South.

"I think we got a real shot at Minnesota. I think we got a shot at Colorado, Oklahoma, Massachusetts and Vermont," Sanders said on NBC's "Meet the Press" Sunday. "So we're looking to the future, not looking back."

A Monmouth University poll of the Democratic race published Monday suggested Sanders is in good shape in at least one of those target states, showing that he leads the former secretary of state by 48% to 43% in Oklahoma.